Glass compositions

ABSTRACT

Glass compositions characterized by consisting essentially on a percent by weight basis of 43-63% SiO2, 14-25% Al2O3, 0 to 30% CaO, 0 to 20% MgO, where the total CaO + MgO expressed as CaO mole equivalent is from 10 to 30% by weight and the total alkali metal oxide expressed as Na2O mole equivalent ranges from 5 to 15%. The log viscosity of these glasses at its liquidus temperature is at least 2.3 and the liquidus temperature ranges from about 2,035* to about 2,320*F. Cooling time for these glasses is less than 96 seconds, for example, in the range of 55 to 68 seconds.

United States Patent [191 Grubb et al;

[ Aug. 19, 1975 GLASS COMPOSITIONS [75] Inventors: Everett F. Grubb, Toledo; Erwin C.

Hagedorn, Oregon; Joseph R. Monks, Toledo, all of Ohio [73] Assignee: Owens-Illinois, lnc., Toledo, Ohio [52] US. Cl. 106/52; 106/50 [51] Int. Cl. C03c 3/04 [58] Field of Search 106/52, 50, 39

[56] References Cited UNITED STATES PATENTS 2,210,254 8/1940 Peddrick 106/52 2,478,151 8/1949 Bair 106/52 3,001,881 9/1961 Slayter. l06/52 3,006,775 10/1961 Chen 106/39 DV 3,071,480 l/l963 Searight et a1 106/52 3,374,381 3/1968 Albinak et al. 106/52 FOREIGN PATENTS OR APPLICATIONS 124,553 6/1947 Australia 106/50 1942 Switzerland l06/5O 1940 United Kingdom 106/50 OTHER PUBLICATIONS Yolf, M. B., Fibreglass, in Technical Glasses; London, 1961, pp. 425-428.

Primary Examiner-Walter R, Satterfield Attorney, Agent, or Firm-Richard D. Heberling; E. J. Holler [57] ABSTRACT Glass compositions characterized by consisting essentially on a percent by weight basis of 43-63% SiO 14-25% A1 0 0 to 30% CaO, 0 to 20% MgO, where the total Ca0 MgO expressed as CaO mole equivalent is from 10 to 30% by weight and the total alkali metal oxide expressed as Na O mole equivalent ranges from 5 to 15%. The log viscosity of these glasses at its liquidus temperature is at least 2.3 and the liquidus temperature ranges from about 2,035 to about 2,320F. Cooling time for these glasses is less than 96 seconds, for example, in the range of 55 to 68 seconds.

2 Claims, No Drawings GLASS tCUlMlPOSllTlOlWS REFERENCE TO A RELATED APPLICATION The present application is a division of application Ser. No. 512,235 filed Dec. 7, 1965, now US. Pat. No. 3,524,738, granted Aug. 18, 1970.

This invention relates to a process for making a novel glass, to the glass and to articles of manufacture made of the glass.

Various types of commercial glasses are manufactured. E. B. Shand on page 4 of his book entitled Glass Engineering Handbook, Second Edition, published in 1958 by lVlcGraw-lrllill Book Company, lnc., New York, NY. presents the approximate compositions of commercial glasses. it can be noted on that page that there are four types of commercial glasses referred to as soda-lime glasses. These glasses are made for the manufacture of window sheet glass, plate glass, containers and electric lamp bulbs. A recent commercial development is the manufacture of a glass called float glass. it is also a sodalime-silica glass and the composition is approximately that of a plate glass.

The main constituents of these sodalime-silica glasses are silica (SiO soda (Na- O) and a mixture of the alkaline earth metal oxides, i.e., calcia (Ca()) and magnesia (MgO). A small amount of the alkali metal oxide content may be potassia (K These glasses always contain a small amount of alumina (A1 0 because some of the batch materials, particularly sand, contain a small amount of alumina. Although the maximum indicated alumina content is 2.5% by weight of the glass, some commercial glass for containers has as much as 9.5% A1 0 by weight of the glass, when the sand of the batch material has a relatively high alumina content. Also, sometimes additional alumina in the form of soda feldspar is incorporated in the batch when the sand is rather low in alumina content, so that the glass will contain several percent of alumina because its presence decreases the rate of devitritication and provides improved durability to the glass.

The conventional batch materials for soda-lime-silica glass utilizes sand, soda ash and dolomite lime. in the reference book entitled Handbook of the Glass industry" compiled and edited by S. R. Scholes and published in 1941 by Ogden- Natney Publishers, llnc., New York, NY a batch for container glass is presented on page 10. The tabulation indicates that borax and feldspar in minor amounts are included in the batch to provide in the glass composition a small amount of boria (B 0 and alumina. The theoretical composition for the glass contains less than 1% of boria and alumina and ignores the alumina content in the sand and the limestone or dolomite used. Various other materials may be present in the batch to provide a fining action or to provide a desired color to the glass. These are used in very minor amounts. The color of the glass that is produced depends on whether the glasses are made under oxidizing conditions or reducing conditions. These materials are disclosed on page 13 of the reference book by Scholes.

Another commercial glass listed on page 4 in the book by Shand, mentioned above, is designated alumi nosilicate glass. This glass contains on a weight basis 57% silica, 20.5% alumina and alkaline earth metal oxides, specifically 5.5% caicia and 12% magnesia, and thus it is an alkaline earth aluminosilicate. it is further indicated that this commercial glass contains 1% soda caAizsnog).

and 4% boria. On page 7 of Shands book it is stated that aluminosilicate glass contain 20% or more of alumina and smaller amounts of calcia or magnesia, that this glassuses a relatively small amount of boria as a flux, and that a very small amount of soda or potassia is present. Such glasses are more difficult to melt and to work than borosilicate glasses. The aluminosilicate glasses are characterized by Shand as having high softening temperatures. On page 6 of his book Shand indicates that borosilicate glasses require higher melting temperatures than soda-lime-silica glasses and are generally more difficult to fabricate into glass articles than is the casewhen using soda-lime-silica glass. On page 161 of his book Shand indicates for four types of glasses the temperature that must be reached by the surface portion of the glass in a tank during melting. For the soda-lime-silica glass this temperature is indicated to be 1,500C. (2,732F.), whereas for the alumi' nosilicate glass it is 1600C. (2912F.).

As mentioned above, soda feldspar may be used in a small amount in making a container glass. lit can be used as a batch material in the manufacture of the alkaline earth aluminosilicate glass; however, because of its high soda content, soda feldspar may be used only as a minor part of the source for the alumina content of this aluminosilicate glass. The main use for feldspar is as one of the raw materials for the manufacture of classic ceramics. This use is described in chapter 2 of the book entitled Ceramics by P. W. Lee and published in 1961 by Reinhold Publishing Corp, New York, NY.

P. W. Lee on pages 16 through 19 describes the various feldspar-s and their source along with a description of nepheline syenite which is also used in the manufacture of classic ceramics, especially white wares. The natural feldspars are indicated to be soda feldspar (NaAlsi O potash feldspar (KAlsi O hereinafter referred to as potassia feldspar, and lime feldspar Nepheline syenite is indicated as comprising by weight about 50% soda feldspar (sometimes called albite), 25% potassia feldspar (sometimes called microcline or orthoclase) and 25% of the feldspathoid called nephelite ornepheline (hla Al- Si Ogl. Theoretical compositions of the feldspars and nephelite are presented on page 16 of the book by Lee who indicates that the feldspars used commercially are commonly intermediate in composition because the pure minerals do not occur in nature in sufficient commercial quantities. For example, potassia feldspars' always contain some soda feldspars and the soda feldspars also contain some potassia feldspar. Furthermore, the feldspars used commercially contain minor amounts of silica, iron oxide and magnesia. The feldspars and the nepheline syenite are used in the manufacture of classic ceramics by serving as fluxes to provide the glassy phase of the ultimate ceramic bodies or articles.

The feldspars make up about 60% of the igneous rocks. They can intercrystallize with each other as well as form an infinite range of solid solutions among themselves so that there is a wide range of the compositions of the feldspars obtained from different deposits. Moreover, it has become common practice to regard any alkali or alkaline earth aluminosilicate mineral as a feldspar type. lln this group of minerals can be included the lithium spars, such as spodumene and petalite, the feldspathoids, such as nepheline, and aplite. These can also intercrystallize among themselves and from a wide variety of solid solutions. They may also contain excesses of one or more oxides from the same family as the principle oxides of the mineral. Thus, there can be a very wide range of compositions in the ore from different deposits.

Nepheline syenite has been used as one of the components of a batch for making glass to be used in the manufacture of glass containers. This is disclosed in U.S. Reissue Pat. No. 25,456 in Table I where it constitutes a minor proportion of the overall batch. The major components are sand, soda ash, and lime or dolomite. Table H of the patent shows that the alumina content for the specific glasses would be substantially less than 3% by weight. Also part of the alumina content is due to alumina impurity in the sand.

U.S. Pat. No. 2,581,638 relates to the manufacture of sheet glass. It discloses a batch in which aplite is present as one of the ingredients. However, the batch contains a large weight each of sand and soda ash, and the total weight of limestone and dolomite exceeds the weight of aplite. As a result, the alumina content of the ultimate glass is only 3.04%, as indicated.

A typical analysis of nepheline syenite on a percent by weight basis is as follows:

SiO 60.2 A1 23.4 CaO 0.3 Na O 10. K 0 5. Iron oxide, expressed as Fe O 0.07 Glass make factor 98.7

Si0 77.7 A1 0,, 16.2 Li O 4.2 Na O 0.4 K 0 0.2 Iron oxide, expressed as Fe O 0.03 Ignition loss at l000C. l.

Trace impurities Spodumene is Li O. A1 0 4SiO which is also designated as being LiAlSi O For this mineral, ignoring impurities, spodumene contains on a weight basis 64.6% SiO 27.4% A1 0 and 8% Li O. Commercially available spodumene had the following typical analysis on a weight percent basis:

SiO 63.4 A1 0 26.2 Li O 7.4 N3 0 1.0 K 0 0.7 Iron oxide, expressed as F6203 0.7 Manganese, expressed as MnO 0.2 Ca0 MgO Ignition loss at l000C.

Aplite is commercially available in a low iron content form, which is suitable for use in the batch for making flint container glass, and in a high iron content form,

which is suitable for use in the batch for making colored container galss. These forms of aplite have the following typical analyses on a Weight percent basis:

In French Pat. No. 1,329,124 and South African Pat. No. 62/2353 it is stated that sodium aluminosilicate glasses must contain at least alumina by weight for the glass, after being subjected to an ion exchange treatment for replacement of sodium ions in the surface layer with potassium ions, to have an increased flexural strength or modulus of rupture that will be substantially retained after considerable abrasion of the glass surface. These sodium aluminosilicate glasses are stated as consisting essentially of, on a weight'basis, at least 5% Na O, at least 5% A1 0 and the balance being SiO but optionally up to about of other compatible glass-forming ingredients may be present. It is further stated that such glasses in which either. the Na O or A1 0 content is above about are generally too low in chemical durability and/or too difficult to melt to be of practical interest. Also it is stated that, as either the Na O or A1 0 content is decreased, the degree of strengthening within a given time diminishes and thus their combined content Should ordinarily not be less than about 15% if substantial abraded strength is to be imparted to the glass by the ion exchange of potassium for sodium in the surface layer of the glass. It is preferred that the Na O content be at least 15% by weight and the Na O content be about 15-16% as optimum.

French Pat. No. 1,329,124 states that, whereas a substantial increase in abraded strength of a sodium aluminosilicate glass may'be observed by effecting a given amount of ion exchange to a given depth, the same is not true for an equal or even greater degree of ion exchange in a soda-lime-silica glass. The patent application further states that with a sodium aluminosilicte glass having an alumina content of less than 10% by weight the rate of ion exchange and sodium replacement becomes too low to be of interest, whereas when the alumina content is increased the sodium ion replacement increases so that the depth of the ionexchanged surface layer increases at a proportionally greater rate. For this reason, the application states that preferably the alumina content is maintained between 16% and 22% by weight. It further indicates that other compatible metal oxides may be incorporated for secondary purposes and may be tolerated in minor amounts. Thus, in general, the total of alkaline earth metal oxides, particularly CaO and MgO, should not exceed 5% by weight and preferably should be no greater than 2-3% by weight based on their total content in the glass.

Conventional soda-lime-silica batch for the manufacture of glass containers, for example, begins to melt at about 800C. (about l472F.). Long after melting begins and even after the batch reaches the desired maximum temperature required for complete melting and reaction for molten glass formation, there is a considerable evolution of gas. This effect requires a substantial period of time to remove the gas bubbles in the fining operation.

As indicated above, alkaline earth aluminosilicate glasses require a fairly high temperature for melting a batch to form this glass and thus either special refractories in the furnace are required or a short life of the furnace lining results.

It is an object of the present invention to provide a process for making glass and products or articles of manufacture from the glass so that the glass in the molten form has improved characteristics or properties as compared with conventional soda-lime-silica glasses and with conventional alkaline earth aluminosilicates.

It is another object of this invention to provide a process for making glass that can be ion exchanged to provide a substantial increase in strength which is retained after an abrasion to a greater extent than is the case with ion-exchanged conventional soda-lime-silica glasses.

Still another object of the invention is to provide a process for making a glass that is more readily ion exchanged than conventional soda-lime-silica glass and is similar in the case of ion exchangeability to those glass compositions that are high in alumina and soda content, i.e., sodium aluminosilicates, and are devoid of or contain at best a very low content of alkaline earth metal oxide such as described in French Pat. No.

1,329,124, as mentioned above.

A further object of this invention is to provide a process for making a glass from a novel batch composition so that the viscosity-temperature relationship and other characteristics of the glass are more desirable than conventional alkaline earth aluminosilicates and at least as good in working characteristics, as a result of the viscosity-temperature characteristics, as conventional soda-lime-silica glasses.

Other objects of the present invention are the glass compositions resulting from the process of the present invention and the articles of manufacture obtained from these glasses especially by the use of conventional automatic machinery, such as that used. for press forming and for molding by either the press-and-blow technique or the blow-and-blow technique, as well as that used for the manufacture of other articles such as glass tubing and rod, plate glass, sheet glass, float glass and glass for electric lamp bulbs.

These and other objects of the present invention will be apparent to one skilled in the art from the description that follows.

The invention, expressed as the process embodiment, is melting and reacting, to form a glass, a mixture consisting essentially of at least one alkali aluminosilicate mineral as a major ingredient and at least one alkaline earth material selected from the group consisting of alkaline earth metal oxide, alkaline earth metal carbonate and alkaline earth aluminosilicate mineral as a minor ingredient, preferably said alkali aluminosilicate mineral major ingredient containing a substantial soda content and said alkaline earth material containing a substantial content of calcia, magnesia or total magnesia and calcia, whereby the glass that is formed has the theoretical composition expressed below. Accordingly, the mixture can contain at least one l alkali feldspar, such as soda feldspar, potassia feldspar, and feldspars that are considered as containing both soda and potassia feldspars (such as soda orthoclase and anorthociase described by Lee on pages 16 and 17 in his book mentioned above), (2) nepheline syenite, (3) nepheline, (4) aplite, and (5) lithium spars, such as spodumene and petalite. The choice of these alkali aluminosilicate minerals to make a specific glass within the present invention is dependent upon where it is desired to make a glass in which the alkali metal is primarily or entirely sodium, lithium or potassium. It is preferred that the mineral or minerals to be used provide sodium as the predominant alkali metal in the glass, although as seen below in examples of the glasses of the invention substantial potassium expressed as K 0 can be present in this preferred embodiment. When it is desired to ion exchange the glass, as explained in more detail later, the replace one alkali metal ion by another alkali metal ion having a larger or smaller ionic diameter, the initial mole content in the glass of the replacing alkali should be substantially less, e.g., less than 75%, of the mole content of the alkali alkali metal ion to be replaced. Also, in the preferred embodiment, when the glass and articles therefrom are not later ion exchanged but it is desired to provide a glass containing soda although potassia may be present, the soda: potassia content on a weight basis is less than 1:1, such as a maximum of 0.75:1. In this case, nepheline syenite and certain feldspars are available in the United States, e.g., Spruce Pine feldspar and Western feldspar, can be used. As seen from the typical analysis of nepheline syenite presented above and these two feldspars presented below, this requirement is met, and Western feldspar being a high-soda feldspar, has a very low K 0 content. When the minor ingredient uses alkaline earth orthosilicate mineral, it is preferably only a partial replacement for alkali earth metal oxide or alkaline earth metal carbonateor mixtures of oxide and carbonate.

The mixture of batch material used in the process of the present invention may contain minor amounts of other compatible materials, such as fining agents and the colorant metal oxides, the latter usually being incorporated in glass for the purpose of providing a specific color to the glass or for providing specific light absorption characteristics. The total amount of such other materials in the batch mixture is a maximum of 5% by weight of the total weight of said major and minor ingredients other than the weight in the batch of the CO content in the mixture when carbonate is used to provide at least part of the alkaline earth metal oxide content.

In view of the fact that the alkali aluminosilicate minerals and any alkaline earth aluminosilicate minerals used in the batch of the present invention will generally contain an amount of iron oxide impurity so that the resultant glass has a color, and thus is not suitable as a flint glass, the present invention is more suited to the manufacture of colored glasses and articles of such glasses.

In view of the foregoing, the batch of the present invention consists essentially on a weight basis of about 55% to about alkali aluminosilicate mineral and about 10% to about 45% of an alkaline earthmaterial as oxide, carbonat and aluminosilicate mineral or mixtures thereof, said carbonate being used to provide said weight percent of oxide, in which the alkaline earth is calcia, magnesia and mixtures thereof. When none of the alkaline earth material is an alkaline earth aluminosilicate, the batch consists essentially on a weight basis of about 75% to about 90%, preferably about 80 to about 95%, alkali aluminosilicate mineral and about 10% to about 25%, preferably about 10% to about 20%, alkaline earth material. As stated above, the alkali aluminosilicate mineral or mixture of such minerals has in the preferred glass a greater soda than potassia weight content.

The preferred glasses of this invention resulting from the melting and reacting of the batch mixture of raw materials, as expressed above, have the following theoretical composition expressed in ranges on a weight percent basis, wherein the glasses consist essentially of expressed oxides and any compatible materials present are limited to by weight of the total content of oxides expressed below:

expressed as Na O However, being within these general ranges does not provide a glass having suitable working characteristics etc. Accordingly, an additional limitation is the log viscosity at the liquidus temperature of the glass, i.e., the logarithm (to the base of the viscosity in poises at that temperature shall be at least 2.3, and preferably at least 3.4 and it is especially preferred that it be at least 4. For this reason there are presented above preferred ranges and especially preferred ranges for the oxides of the composition, but again being within these ranges do not necessarily give a glass with the minimum log viscosity at the liquidus that is desired.

The preferred glass of the present invention, as will be apparent from Table I that is presented later, has the following typical properties that are obtained by the proper choice of the specific weight percent of the raw or batch mterials:

Typical properties Liquidus (F.) 2035 2320 Log viscosity at liquidus at least 2.3 Log viscosity of 2 at "F. 2345 3080 of 3 at F. 2065 2600 of7atF. 1521-1736 Cooling time Sec. 55 68 Thermal expansion As a glass is cooled from the temperature at which its viscosity is suitable for working, the sooner it reaches a temperature at which it has a viscosity at which the article will not sag, the shorter is the time that it should be retained in a mold for most efficient use of the mold. Thus, the shorter the cooling time the faster can be the rate at which the glass articles are made. The term setting rate is a term that is used as an equivalent for the term cooling time.

During the past 25 years glass compositions for the manufacture of glass containers have become faster and faster setting glasses. The cooling times have been reduced from about 108 to 96 seconds, and automatic machinery speeds have increased with these decreases in cooling time. Accordingly, from this standpoint the glasses of the present invention are a continuation of this desirability of reduction in cooling time of glass used for automatic machinery manufacture of glass articles such as glass containers. For the maximum utilization of the cooling time characteristic of the glasses of this invention, it would be desirable to develop new automatic machinery that can utilize the higher setting speeds of the compositions of this invention. This does not imply that present equipment is not suitable for the manufacture of articles such as bottles using glasses of the invention. As a matter of fact, two compositions have been used to form suitable bottles on a single IS machine used for development work. Although one of them was so fluid at temperatures down to the devitrification point, this glass was puddled into the blank and then the conventional procedure was followed for the manufacture of the containers. The other glass composition was successfully used to form 500 glass containers by using a temperature about F. lower, from a viscosity standpoint, than that used for the conventional soda-lime-siliea glass used in this development machine for the studies of improved construction of the machine. It is believed further, that by changing the orifice ring size of the feeder for this machine, higher glass temperatures could have been used and this composition would be expected to be useful with the standard feeder and the standard bottle-forming machines with their conventional operating conditions, at least with better cooling of the mold equipment.

Five hundred suitable containers were made in the following manner. A glass melting tank lined with conventional zircon refractory was used to make glass from an approximately 1,600-pound batch having the following ingredients:

Spruce Pine feldspar (1) 1328.0 lbs. Raw dolomite limestone (2) 520.0 lbs. Arsenic trioxide (3) 4.0 lbs. Niter (4) 4.0 lbs. Sodium antimonate (5) 4.8 lbs.

1. The analysis of this feldspar, on a weight percent basis, is as follows:

The glass made factor is 99.9%

2. The analysis of this material on a weight percent basis is as follows:

sio 0.5 A120,, 0. l mo, 0.072 52 8'8? CaO 0.04 CaO 30.4 MgO 2 1 .5

The glass made factor is 52.6%, as the rest is CO of the carbonate.

3. Used as a fining agent.

4. Used as an oxidizing agent 5. Used as a fining agent.

This glass has a liquidus of 2,245F. and a log 4 viscosity of 2,060F. The theoretical glass composition, on a weight percent basis, is as follows:

Si 2 3 CuO MgO Na O K 0 As,O 5 Sb Q, 3 5: 0,, 0.09

To make this glass the furnace was preheated to 1,350F. and then the main burners were fired. The batch was charged at the rate of 200 lbs/per hr. About three hours later glass to a depth of 6 inches was present in the melter. The charging of batch was ceased until the batch now in the melter became completely molten. Then the charging of the batch was restarted at a rate' of about 150 lbs/per hr. When molten glass came through the orifice a pull rate of 70 lbs/per hr. was maintained. Eventually the desired glass depth of about 1 1 inches in the melter was reached. The charger was switched to automatic control. The glass in the melter was found radiamatically to have a temperature of 2,725F. The glass in the refiner was at 2,300F. and in the bowl of the feeder was at 2,200F., while the emerging stream of glass from the orifice was at 2,190F. The glass devitrified when the stream on cooling reached 2,l70F. The pull rate was reduced to 50 lbs./per hr. As a result, the emerging glass stream was at 2,210F. and parisons were formed, but the glass set too fast to reshape the parison in the blank mold.

Instead of the expected liquidus of 2,245F. and log 4 viscosity of 2,060F., the liquidus was 2,300F. and the log 4 viscosity was 2,025F. It was believed that the parisons could be formed into glass containers by the addition of a large amount of heat to the blank molds, but it was decided to change the glass composition. To accomplish this, the tank was drained until only six inches of molten glass remained in the tank. A new approximately l,600-pound batch having the following ingredients was charged to the tank at about 300 lbs/per hr. while glass was pulled at about 90 lbs/per hr. until the desired glass level was reached.

Niter (4) See notes above for previous batch for items (I) through (5).

6. This limestone had the following'composition in percent by weight:

CaO 55.3 MgO 0.4 2 0.4 A1 0, 0.04 Pep, 0.046 so 0.03 P205 0.008 CF20:

Glass made factor is 56.3%, as rest is CO as carbonate.

The theoretical composition of this glass in percent by weight is as follows:

sio 563 A1 0, 15.6 CaO 14.8 Na O 5.5 K20 3.7 MgO 3.5 A5202: Sb Q, 0.23 re o 0.08

Later the pull was reduced to lbs/per hr. The pull was not reduced further because it was being used to change the composition in the tank, and fining of the glass was not the immediate objective. The temperature in the feeder bowl and in the refiner were reduced, and eventually the pull rate was reduced to about 25 lbs/per hr. in order to aid refining of the glass. When the glass had been refined fairly well, the production of the 500 sample containers (GB-2000 bottles) was started with a refining temperature of 2,300F., a bowl temperature of 2,230F. and a stream temperature of 2180F. Four bottles per minute were made, and these were satisfactory except when external heating being applied to the blank mold was removed. The desired 500 bottles were obtained satisfactorily and the experimental production was ceased. At the end of the operation of the glass being pulled did not have the composi tion of the modified composition indicated above. Rather, it was between it and the first composition. The liquidus temperature of the pulled glass was 2,255F. which was adequate to permit a gob to be formed without devitrification.

A number of glasses of the preferred type of the present invention were prepared along with a number of glasses that do not have the properties required, and all of these were made using Spruce Pine feldspar, Nepheline syenite or Western feldspar as the alkali aluminosilicate and high-calcite Mississippi limestone, raw dolomite lime and mixtures of these limes as the alkaline earth material. All of the glasses were made by melting the mixture of batch ingredients in platinum crucibles at a temperature of 2,700 to 2,800F. The time at the melting temperature was about 18 to 20 hours from the time that the last of a batch mixture was added to the crucible. The platinum crucibles were kept at this temperature for melting the glass by being placed in either a gas-fired furnace or an electric furnace. There was no difference as to the type of furnace with regard to the quality of the final glass.

The batch contained sufficient high-calcite Mississippi limestone to provide a specific amount of calcite lime, in view of the fact that the limestone is the carbonate and the lime is the oxide. Similarly, the amount of raw dolomite limestone was chosen to provide a specific weight percent of dolomite lime. The analyses of these limestones have been presented above.

The analysis of the nepheline syenite used is presented above. A typical analysis of Spruce Pine feldspar on a weight percent basis is as follows:

Chromium oxide expressed,

as Cr O Glass made factor 99.87%

The Western feldspar has a typical analysis on a weight percent basis as follows:

- Continued Chromium oxide expressed, 0.0003

as cr o. P 0 0.064

Glass made factor 99.4%

The liquidus temperature is a term that has a well known meaning in the art and need not be defined. The coefficient of expansion is another property indicated, and its meaning is stated at the end of Table 1. The chemical durability of the glass is expressed as numerical values where such durability has been indicated. The specific tests have no absolute commercial meaning and cannot be exactly correlated with the chemical resistance of glassware under particular service conditions. However, the lower the value for a glass, the more likely it is that the products made of this glass will be more resistant to such liquid environment than a glass having a higher value in the durability test. In Table I most of the compositions are presented based on theoretical values in view of the known compositions of the batch ingredients. Thirteen of the glasses were analyzed. A comparison of the actual versus theoretical oxide compositions indicated satisfactory agreement between the two sets of values. Analyzed compositions are distinguished by means of the asterisk for the silica content. For comparison, a typical amber container glass has 2 3 19.1 o i oxide expressed, (H46 21 llqUldLlS of 1900 F. and log viscoslties of 2, 3 and 7 as 2 3 at 2670", 21 85 and 1405F., respectively. The percent E33 8': Na O dissolved in chemical durability tests using H O 2 0.6 and N/50 H SO were 0.027 and 0.032, respectively.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Compositions in Weight Percent Oxide composition SiO,, 1 48.22 54.44 58.32 61.22* 57.94* A1 0 13.38 15.13 15.97 16.57 16.34 CaO 30.23 21.24 16.03 11.67 14.30 MgO 0.22 0.18 0.16 0.04 1.33 Na O 4.72 5.35 6.14 6.11 5.96 K 0 3. 17 3.59 3.32 4.08 4.00 rc o 0.06 0.07 0.06 0.07 0.1 1 Liquidus, F. 2400 2320 2285 2245 2200 Log vis. at liquidus 2.00 2.80 3.29 4.12 3.61 Log vis. at F.

2 2400 2615 2850 3080 2815 2.5 2415 2585 2815 2570 3 2260 2380 2600 2380 4 2025 2100 2280 2105 5 1850 1905 2045 1915 6 Y 1725 1770 1870 1775 7 1625 1666 1736 1667 7.65 1613 1571 1612 1663 1610 Cooling time Sec. 62 62 61 a X 10" (0300C.) 86.5 80.1 i 76.5 73.7 76.7**' Batch ingredients by weight) Spruce Pine feldspar 85 Calcite lime 30 20 15 10 1 1.25 Dolomite lime 3.75 Chemical durability Na O dissolved) H O 0.010 0.007 0.003 N/SO u so. 0.19 0.12 0.037 Ex. 6 Ex. 7 Ex. 8 EX. 9 Ex. 10 Compositions in Weight Percent Oxide composition '0 47.69 51.48 54.90 58.29 60.96* A1 0 13.53 14.11 15.04 15.97 17.40 CaO 24.95 20.71 16.88 13.03 9.22 MgO 6.29 5.27 4.23 3.19 1.63 Na O 4.67 5.42 5.78 6.14 6.26 K 0 2.82 2.93 3.13 3.32 4.24 F820;; 0.06 0.08 0.04 0.06 0.1 1 Liquidus, F. 2295 2250 2160 2180 2175 Log vis. at liquidus 2.13 2.54 3.34 3.73 4.39

Log vis. at "F.

TABLE 1 -Cont1nued Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10' Compositions in Weight Percent Oxide composition 2 2340 2425 2605 2800 2.5 2185 2260 2405 2570 2810 3 2065 2130 2245 2385 2595 4 1875 1925 2005 21 15 2270 5 1740 1780 1835 1920 2045 6 1640 1670 1710 1780 1875 7 1599 1582 1610 1668 1742 7.65 1516 1535 1557 1608 1672 Cooling time Sec. 54 56 62 62 61 a X 10 (-300"C.) 80.7 78.9 75.7 77.8 72.3** Batch ingredients (71. by weight) Spruce Pine feldspar 70 75 80 85 90 Calcite lime 15 12.5 10 7.5 Dolomite lime 15 12.5 7.5 5 Chemical durability (7L- Na O dissolved) H O 0.012 0.007 0.003 N/SO 1-l,,SO 0.19 0.13 0.053

Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Compositions in Weight Percent Oxide composition '0 51.50 54.86 58.76* 40.87 47.66 A1 0; 14.11 15.04 14.15 11.40 13.28 CaO 18.21 14.88 10.60 24.42 18.72 MgO 7.79 6.26 3.18 16.54 12.44 Na O 5.42 5.78 8.59 4.01 4.68 K 0 2.93 3.13 4.24 2.69 3.15 F0 0; 0.04 0.05 0.08 0.07 0.07 Liquidus, F. 2195 2295 2100 2390 2315 Log vis. at liquidus 2.68 2.90 3.88 1.22 2.0 Log vis. at F.

2 2410 2610 2750 2135 2300 2.5 2245 2420 2525 2015 2160 3 21 15 2265 2345 1920 2050 4 1915 2040 2075 1770 1900 5 1775 1875 1880 1665 1795 6 1665 1750 1735 1585 1715 7 1580 1653 1624 1520 1660 7.65 1534 1600 1564 1486 1629 Cooling time Sec. 55 55 67 48 a X 10" (0--300"C.) 80.7 66.3 83.1** 79.8 78.7 Batch ingredients by weight) Spruce Pine feldspar 75 80 85 60 70 Calcite lime 6.25 5 3.75 Dolomite lime 18.75 15 11.25 40 30 Chemical durability N3 0 dissolved) H O 0.033 0.019 N/SO H Neutral 0.21

Ex. 16 Ex. 17 Ex. 18 Ex. 19 Compositions in Weight Percent Oxide composition '0 51.45 54.46 58.26 6156* A1 0 14.11 15.16 15.98 17.40 CaO 15.71 13.02 10.02 7.04 MgO 10.32 8.33 6.22 3.55 N3 0 5.42 5.36 6.14 6.26 K 0 2.93 3.60 3.32 3.95 Fe Q, 0.07 0.07 0.06 0.1 1 Liquidus, F. 2270 2250 2215 2205 Log vis. at liquidus 2.47 3.0 3.69 4.31 Log vis. at F.

6 1665 1720 1790 1885 7 1573 1620 1677 1752 7.65 1523 1565 1615 1679 Cooling time Sec. 58 60 63 58 a X 10 (0--300"C.) 74.6 73.0 71.2 69.6** Batch ingredients by weight) Spruce Pine feldspar Calcite lime Dolomite lime 25 20 15 10 Chemical durability Na O dissolved) H O 0.007 0.003 N/50 H SO 0.19 0.057

TABLE I Contmued Ex. 20 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Compositions in Weight Percent Oxide composition SiO 42.77 48.82 51.84 5504* 5194* A1 16.57 18.92 20.12 20.38 18.87 CaO 29.73 19.92 15.01 10.06 13.38 MgO 0.21 0.14 0.08 0.00 1.68 M 0 7.07 8.08 8.59 9.56 9.45 K 0 3.57 4.04 4.29 4.50 4.30 Fe o 0.08 0.08 0.0 7 0.06 0.10 Liquidus, F. 2145 2255 2195 2140 21 15 Log vis. at liquidus 2.48 2.78 3.41 4.18 3.72 Log vis. at F.

2 2290 2525 2720 2935 2715 2.5 2140 2345 2495 2690 2490 3 2020 2195 2315 2485 2315 4 1840 1970 2050 2185 2050 5 1715 1805 1865 1965 1865 6 1620 1680 1725 1800 1725 7 1547 1581 1614 1668 1615 7.65 1508 1528 1556 1598 1556 Cooling time Sec. 53 63 66 64 64 a X 10 (0-300"C.) 95.2 90.9 88.6** 85.2 86.9** Batch ingredients by weight) Nephelinc syenite 7O 80 85 90 85 Calcite lime 20 15 10 1 1.25 Dolomite lime 3.75 Chemical durability Na O dissolved) H O 0.029 0.018 0.006 N/ H 80 0.21 0.16 0.13

Ex. 25 Ex. 26 Ex. 27 Ex. 28 Ex. 29 Compositions in Weight Percent Oxide composition 0 54.38 42.61 45.75 48.64 51.79* A1 0 21.00 16.36 17.78 18.69 20.12 CaO 9.58 23.86 19.81 16.03 12.01 MgO 0.61 6.25 5.19 4.17 3.11 Na O 9.76 7.14 7.58 8.16 8.59 K 0 4.53 3.71 3.79 4.24 4.29 Fe O 0.06 0.07 0. 10 0.07 0.08 Liquidus, F. 2035 2310 2275 2195 2090 bog vis. at liquidus 4.68 1.97 2.35 2.98 3.80

bog vis at F.

2 2955 2300 2390 2525 2710 2.5 2705 2150 2230 2340 2485 3 2500 2030 2100 2190 2310 4 2190 1840 1895 1970 2045 5 1970 1705 1745 1815 1860 6 1805 1600 1635 1700 1720 7 1672 1521 1542 1608 1612 7.65 1601 1476 1493 1560 1555 Cooling time Sec. 68 58.5 62 57 65.5 a X 10 (0-300"C.) 86.6** 90.8 88.2** 87.0 86** Batch ingredients by weight) Nepheline syenite 90 Calcite lime 7.5 15 12.5 10 7.5 Dolomite lime 2.5 15 12.5 10 7.5 Chemical durability M 0 dissolved) H O 0.020 0.010 N/50 H 50 0.21 0.16

Ex. 30 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Compositions in Weight Percent Oxide composition '0 5426* 42.72 45.75 48.77 51.80 A1 0 20.65 16.61 17.78 18.95 20.12 CaO 8.00 20.73 17.31 13.92 10.51 MgO 2.47 9.27 7.71 6.18 4.63 Na O 9.86 7.07 7.58 8.08 8.59 K 0 4.53 3.54 3.79 4.04 4.29 F6203 0.06 0.06 0.08 0.06 0.06 Liquidus, F. 1990 2280 2260 2210 2155 Log vis. at liquidus 4.92 2.0 2.33 2.87 3.49 Log vis. at F.

2 2280 2370 2495 2665 2.5 2720 2125 2210 2320 2460 3 2510 2005 2080 2175 2290 4 2195 1820 1875 1955 2040 5 1975 1690 1730 1790 1860 6 1810 1590 1615 1665 1720 7 1683 1516 1527 1570 1617 7.65 1615 1474 1478 1517 1560 Cooling time Sec. 67 57 62 63 64 a X 10 (0300"C.) 85.7** 87.3 86.5** 84.3 84.3** Batch ingredients by weight) Nepheline syenite 7O 75 80 85 Calcite lime 5 7.5 6.25 5 3.75 Dolomite lime 5 22.5 18.75 15 11.25

TABLE 1 Continued Oxide composition Compositions in Weight Percent Chemical durability ("/6 N320 dissolved) H O N/50 H SO Oxide composition CaO MgO

Na O

2 8 Liquidus, F. Log vis. at liquidus Log vis. at F.

7.65 Cooling time Sec. 0: X 10 (0300"C.) Batch ingredients (7(- by weight) Nepheline sycnitc Calcite lime Dolomite lime Chemical durability N320 dissolved) Oxide composition MgO Na O

Fe O Liquidus, F. Log vis. at liquidus Log vis. at F.

7.65 Cooling time See. a X 10 (0-300C.) Batch ingredients by weight) Western feldspar Calcite lime Dolomite lime Chemical durability Na O dissolved) N/5O H 50 Oxide composition Fe O Liquidus, F. Log vis. at liquidus Log vis. at F.

0.1 1 Ex. Ex. 36 Ex. 37

Compositions in Weight Percent 0.014 0.012 0.23 0.21 Ex. 40 Ex. 41 Ex. 42

Compositions in Weight Percent Ex. 45 Ex. 46 Ex. 47 Compositions in Weight Percent Ex. 38 Ex. 39

Ex. 43 Ex. 44

0.010 0.007 0.19 0.13 EX. 48 Ex. 49

TAB LE l- Continued Ex. 45 Ex. 46 Ex. 47 Ex. 48 Ex. 49 Compositions in Weight Percent Oxide composition Cooling time Sec. 66 56.5 54 60 62 a X (()-30OC.) 73.2 75.1 76.6 73.3 70.0 Batch ingredients by weight) Western feldspar 9O 8O 7O 80 9O Clacite lime S 5 Dolomite lime 5 30 10 Chemical durability Na O dissolved) H O 0.003 0.024 0.010 0.004 N/50 H 50, 0.036 0.22 0.21 0.051

a means the average coefl'lcient of lineal thermal expansion per C.. c.g., glass of Example 1 has an average coefficient of 86.5 X l0' /C. over the range from 0 C. to 300C.

It will be apparent from a comparison of the values expressed in Table I that some of the ranges of ingredients for the compositions of the present invention can be met by specific example glasses which are actually examples of those glasses not suitable and not part of the present invention. For example, glass No. 31 falls within the general ranges of the preferred glass of the present invention in all respects, considering that the silica content of 42.72% is substantially 43%, the lower end of the general range. However, it is to be noted that the log viscosity at the liquidus temperature is lower than the minimum required end, of course, much lower than the minimum required for the preferred glass, which is a value of at least 3.4, as expressed above. In view of the foregoing general range, preferred range and especially preferred range for the compositions and the minimum log viscosity at the liquidus of 2.3, 3.4 and 4, respectively, glass numbers 1, 6, ,14, 15, 26, 31, 35, 40, 43 and 47 are outside the scope of the glass of the preferred embodiment. Glass numbers 4, 10, 19, 23, 25, 30, 39, 42, 45 and 49 are examples of glasses in the especially preferred range. The glasses numbers 5, 9, 13, 18, 22, 24, 29, 34 and 44 are examples of glasses of the preferred range but lacking properties to meet the requirements of the especially preferred range. The glasses not mentioned by number in the foregoing comments meet only the requirements of the general range.

The chemical durability tests were obtained by crushing a glass sample to provide a quantity of crushed glass that passes through a No. 40 sieve and is retained on a No. 50 sieve. Exactly 10 grams of the crushed sample of glass is placed in a 200-ml. Erlenmeyer borosilicate flask that' had been previously aged with the attacking liquid medium for at least 24 hours at at least 90C. Exactly 50 ml. of the attacking liquid medium, either purifled water or 0.02 N H SO aqueous solution, was added. The flask, after being stoppered with a one-hole rubber stopper equipped with a 12-inch length of 3/16- inch chemically resistant glass tubing, wasimmersed in a water bath held at 90C. (within 02C. for 4 hours. Then the flask and its contents were cooled by running tap water on the outside of the flask. Exactly 40 ml. of the aqueous extracting medium in the flask is transferred to another flask.

When the attacking medium is water, the transferred solution is titrated with 0.02 N H 50 to an excess of 1 ml. using two drops of methyl red indicator that are added prior to the start of the titration. Then the flask is heated to boil the contents and swirled. This is repeated three times to remove all dissolved gasses. Thereafter the solution is titrated with 0.02 N NaOH with methyl red to its end point. The percent Na O is determined by multiplying the difference in the volume in milliliters of acid and base used in the two titrations by the factor 0.00775.

In the case of sulfuric acid as the attacking medium, to the cooled aliquot of attacking medium two drops of methyl red indicator solution is added and then the acid medium is titrated with 0.02 N NaOH to the end point. The percent Na O removed by the attacking medium is calculated by multiplying the difference between the number of milliliters of the NaOH solution equivalent to 50 ml. of 0.02 N H SO used and the number of milliliters of the NaOH solution actually used in the titration by the factor 0.0062.

An illustrative glass of the preferred embodiment of the present invention using aplite as the source of alkaline earth aluminosilicate mineral is now described. A batch containing aplite and raw dolomite limestone was melted, in a manner described above for the 49 samples, the the theoretical composition of the glass obtained is as follows:

sio

A1 0, 19.76 010 13.94 Mg 6.24 Na O 5.49 K20 2.15 mo 0.36

to make a glass, and in some earlier compositional data,

The initial work was primarily limited to dolomitefeldspar mixtures corresponding to glass composition Example No. 16 containing 25% R from dolomite. The results reported here were generally obtained on small samples (less than one gram) heated at l0C./min. (l8F./min).

Below about l,l90C. (2,170F.), little or no reaction was observed between the materials; i.e., only reactions which occurred when the materials were heated by themselves were observed. The reactions of the individually heated materials are described below.

The quartz present in the feldspar undergoes the well-known polymorphic transformation at about 575C. (1,070F.).

Small quantities of gas were evolved below l,000C. (1,830F.). This accounts for the ignition loss of 0.14% for this sample. Although this gas evolution has not been investigated in detail, it seems to primarily consist of CO and H 0 evolution. Based on the complex appearance of the curves, there was probably more than one reaction involved.

Melting of the feldspar began at about 1,190C. (2,170F.) and was substantially complete by about 1,300C. (2,370F.). The viscosity of the liquid formed was high so that the entrapped bubbles did not begin to rise to the surface until higher temperatures were reached. Because of the high viscosity, the initial liquid flow and bubble entrapment occurred rather slowly.

The raw dolomite began decrepitating below 400C. (750F.) when heated at lOC./min. The maximum rate of decrepitation occurred at around 600C. l,l00F.) for minus 40 mesh material.

Dissociation occurred in two stages, represented by the following equations:

1. CaMg(CO CaCO MgO C0 2. Ca0 C02 The influence of CO partial pressure on the first stage was small. Small quantities of CO evolution began at 320C. (610F However, the major part of the first stage of dissociation occurred between 700 and 800C. l300-1470F.).

The temperature of the second stage of dissociation was strongly dependent on the CO partial pressure. In flowing N both stages overlapped considerably, whereas in one atmosphere of CO the second stage did not begin until about 900C. (1,650F.). Completion of the second stage was dependent on the experimental conditions (e.g., grain size, sample size, atmosphere and heating rate). When heated at C./min. at one atmosphere pressure, the second stage was virtually complete below 1,000C. (1,830F.) for most conditions.

Little reaction occurred between the dolomite and feldspar until the feldspar began to melt. As the feldspar melted, the dolomite (primarily MgO and CaO) was dissolved in the melt. As the sample melted, the gas present between the grains was entrapped and bubbles were formed.

A small quantity of gas appeared to be evolved from the sample in the form of bubbles as the dolomite went into solution. This gas appeared to be primarily CO However, small quantities of other gases, such as $0 H O, or 0 may also be' evolved. For example, analyses of the seeds in quenched samples generally indicated less than 1% CO As the temperature was increased further and the dolomite went into solution, the viscosity of the melt decreased. As the viscosity decreased, the bubbles began to rise to the surface and burst. The entrapped gases, including the small quantity of CO were released at this time. For composition No. 16 this occurred at about l,250l 300C. (22802370F.). For higher viscosity compositions, it occurred at higher temperatures.

It was decided to compare the melting characteristics of dolomite-feldspar and soda-lime batches. Both samples were minus lOO mesh, but the weight of dolomitefeldspar sample used was about five times the weight of soda-lime sample used. Otherwise, the runs were identical.

Asindicated by an effluent gas analysis, melting of the dolomite-feldspar sample began at about 1,200C. (2,200F.). Over 99.99% of the gas evolution was completed before melting began. The only gas detected after melting was the small quantity of CO released as the bubbles burst at around l,270C. (2,318F.).

Melting of the soda-lime batch began at around 800C. (1,470F.). Even though a smaller sample was used, considerable gas evolution was observed long after melting began. Although both batches entrap furnace atmosphere as they initially melt down, considerable amounts of additional gases were evolved from the sample itself in the case of the soda-lime sample.

The limited observations made to date suggested that the dolomite-feldspar batch become batch-free at a lower temperature than the soda-lime batch used to make flint glass. Batch samples were heated at l,450C. (2,640F.) for 10 minutes in air and quenched. Both samples were coarse grained (as received). The sodalime sample still contained numerous grains of unmelted batch, while the dolomite-feldspar sample was much nearer being batch-free.

There were two features of the melting characteristics of dolomite-feldspar compositions which seemed important:

1. Melting began at relatively high temperature and was completed within a small temperature range.

2. Almost all gas evolution was completed before melting began; consequently, the majority of the seeds consisted of entrapped furnace atmosphere.

These characteristics have several implications:

1. The batch should flow easily up to relatively high temperatures since melting doesnt begin until about 1,200C. (2,200F.)l., This suggested the possibility of transferring waste heat directly to the batch. For example, an estimate based on DTA data indicated that of the heat required-to heat composition No. 16 to l,400C. (2,250F.) was consumed below l,000C. (1,830F.).

2. Since melting occurred rapidly and no lowviscosity liquid was formed, there was little opportunity for segregation during melting.

3. If entrapment of furnace atmosphere during melting could be avoided, little refining would be needed.

The inducing of surface stress in the glass of the present invention by potassium for sodium ion exchange to improve modulus of rupture was established. Although these samples represented wide ranges in oxide composition, a preliminary experiment involving all 49 glass samples was conducted.

Each sample was subjected to ionic exchange conditions by immersion of the glass in molten KNO The potassium nitrate was held at 750F. for a period of 20 hours. This preliminary experimentation indicated the ion exchange increased as the percent RO (Ca MgO) decreased. Also, these data indicated that the CaO was more detrimental than the MgO.

On the basis of these exploratory results seven of the especially preferred glass compositions were selected for detailed study. These compositions represented glasses made from 90% feldspar or nepheline-syenite and mixed calcite-dolomite limes. Unlike the preliminary studies which were made at 750F., ionic exchange was conducted in molten KNO at 850 and 1050F. Two periods of time were used, namely, minutes and 1 hour. This portion of the ion exchange investigation indicated that higher surface stresses were obtained at the higher temperatures. Also, the depth of ion exchange penetration increased at the higher temperatures.

On the basis of these results glass composition Example No. 39 was selected for more detailed studies. The specific purpose of this study was to determine the relationship between the depth and magnitude of the induced stress with time and temperature. During this specific ion exchange study the following times were utilized: 4, /2, l, 2 and 4 hours. Temperatures applied were 1000", lO50, 1100, ll50, and 1,250F. These temperatures were selected since temperatures above and below the strain point of the glass (1,150F were desired. Unlike the results obtained at the lower temperature interval, the magnitude of the resulting stress appears to decrease as the exchange temperatures increased. In general, the depth of stress was proportional to the square root of time. Also, stress depth increased as the temperature increased until the glass appeared to flow and accommodate the larger potassium ion. The depth in microns (u) and magnitude of the induced compressive stress in thousands of pounds per square inch (Mpsi) are shown in Table 11.

TABLE II A Hr. V2 Hr. 1 Hr. Temp. F. MPSI ,u. MPSI [.L MPSI p.

26 25 19 4O 1 150 ll 35 12 35 1O 68 1250 3 3O 3 80 2 55 2 Hrs. 4 Hrs. Temp. F. MPSI [.L MPSI p 11 83 19 100 1150 6 95 9 130 1250 nil nil Exchange in KNO hath; all others were exchanged in a eutectic mixture by weight of 32.8% NaCl and 67.2% K 50 On the basis of these initial ion exchange studies it appeared that the best exchange conditions for glass composition No. 39 were 1,000 to 1,050F. for 3-4 hours.

Various glasses of the preferred embodiment of the present invention, that are shown in Table I, have been subjected to ion exchange with potassium nitrate at 850 and 1050F. for 15 minutes and 60 minutes. In addition, a glass made from a batch of 90% by weight Spruce Pine feldspar and 10% by weight magnesia has been ion exchanged with potassium nitrate at 1,050F. for 60 minutes. The latter glass had a compressive stress layer of 38 microns. The five-inch rods drawn from this glass in molten state, after the ion exchange, had an unabraded flexural strength of 79,800 psi. After an abrasion with a blast of grit, rods of this ionexchanged glass had a flexural strength or modulus of rupture of 25,600 psi which indicates a substantial retention of flexural strength.

Reissue U.S. Pat. No. 25,456 mentioned above shows a glass in Table III that contains 9.5% alumina. This is a glass made in the West coast using a sand with a highalumina content. Sample rods of this glass, without the colorant and thus a flint glass for container or bottle manufacture, after immersion in molten potassium nitrate at 750F. for 18 hours had a surface compression of 8,000 psi and a compressive surface layer depth of 20 microns.

Various glass compositions of the preferred embodiment of the present invention were immersed in molten potassium nitrate at 750F. for 20 hours, and the maximum compressive stress in the surface layer ranged from 2,000 to 28,000 psi with most of the values being above 10,000 psi. In addition, almost all of the ionexchanged glass rods had compressive stress surface layer depths of at least 40 microns. The best results were obtained when the alkaline earth aluminosilicate mineral was used in an amount of 90% of the batch as compared vwth the use of The glasses of the preferred embodiment of the present invention, i.e., glasses having soda as the primary alkali metal oxide constituent, have been described above in considerable detail, but the glasses in which when potassia or lithia is the primary alkali metal oxide ingredient are within the scope of the present invention, as described earlier. In the case of the glass in which lithia is present in a substantial percentage of the alkali metal oxide content on a mole basis or is the predominant alkali metal oxide on a mole basis, one of natural lithium feldspars is used in the batch, and the lithium ions in the surface layer of the glass produced can be at least partially replaced by sodium ions or potassium ions at an elevated temperature below the strain point of the glass to provide a compressive stress surface layer, as is already described in the prior art in relationship to glasses of other compositions and made by processes other than the process of the present invention.

The predominant alkali metal ion in the surface layer of a glass of the present invention can be replaced by ion exchange with an alkali metal ion having a smaller ionic diameter using ion-exchangeable materials at an elevated temperature in contact with the glass in accordance with procedures already disclosed in the prior art. This is accomplished at an elevated temperature which can be below the strain point initially, but in such a case the product is heated to a temperature above the strain point to heal any minor surface defects, before cooling. Ion exchange is preferably carried out at a temperature above the strain point but below the softening point of the glass.

The glass compositions, of the preferred embodiment at least, have liquidus temperatures that are low as compared with commercial aluminosilicate glasses. The glasses of the present invention, that are shown in Table I, have a log viscosity at the liquidus of at least 2.3 have a maximum of 2,320F., while those that have a log viscosity at the liquidus of at least 3.4 have a maximum liquidus temperature of 2,215F.

Various terms, such as softening point and strain point, have been used. These terms are well known and thus require no definition in this application.

Modifications of the present invention will be apparent to one of ordinary skill in the art. Thus the foregoing description has been presented for purpose of illustration and not for the purpose of limiting the invention which is limited only by the claims that follow.

We claim:

1. A glass having an oxide composition consisting essentially, on a percent by weight basis, of

Si 51 63 AI O l 22 Total Ca0 MgO expressed as CaO mole equivalent [0 22 Na O 3.5 K 0 O 5 Total alkali metal oxide present as Na O or a mixture of M1 0 and K 0 expressed as Na O mole equivalent [5-15] 7 14 2. The glass of claim 1 wherein said oxide composition is as follows:

Si0 54 63 N 0 l7 22 Total Ca0 MgO expressed as CaO mole equivalent l0 12 Total Na O K 0 expressed as Na O mole equivalent 8 l3 and wherein said log viscosity is at least 4. 

1. A GLASS HAVING AN OXIDE COMPOSITION CONSISTING ESSENTIALLY, ON A PERCENT BY WEIGHT BASIS, OF
 2. The glass of claim 1 wherein said oxide composition is as follows: 